LAMMPS

The powder bed fusion (PBF) process is widely
adopted in many manufacturing industries because of its
capability to 3D print complex parts with micro-scale precision.
In PBF process, a thermal energy source is used to selectively
fuse powder particles layer by layer to build a part. The build
quality in the PBF process primarily depends on the thermal
energy deposition and properties of the powder bed. Powder
flowability, powder spreading, and packing fraction are key
factors that determine the properties of a powder bed.
Therefore, the study of these process parameters is essential to
better understand the PBF process. In our study, we developed
a two-dimensional powder bed model using the granular
package of the LAMMPS molecular dynamics simulator.
Cloud-based deposition was adopted for pouring powder
particles on the powder bed. The spreading of particles over the
substrate was mimicked like a powder bed system. The powder
flowability in the proposed study was analyzed by varying the
particle size distribution. The simulation results showed that a
greater number of larger particles in a power sample results in
an increase in the Angle of Repose (AOR) which ultimately
affects the flowability. Two different kinds of recoater
geometry were considered in this study: circular and
rectangular blades. Simulation results showed that depending
on the recoater shape there is a change in the packing fraction
in the powder bed. Cross-sectional analysis of the power bed showed a significant presence of voids when a greater number
of larger particles existed in the powder batch. The packing
fraction of the powder bed was found to be a strong function of
particle size distribution. These analyses help understand the
influence of particle size and recoater shape on the powder bed
properties. Findings from this study help to provide a guideline
for choosing particle size distribution if the spherical particles
are considered. While the present study focuses on the spherical
powder particles, the proposed system can also be adapted to
the study of powder bed with aspherical particles.

A B S T R A C T The paper reports manufacturing of a multi-layer graphene embedded composite of aluminium alloys by direct exfoliation of graphite into graphene with the help of Friction Stir Alloying (FSA). The formation of this nano-composite and optimization of the process parameters led to an approximately twofold increase in the strength, without loss in ductility, due to the dispersion of the graphene in aluminium. The manufacturing process is scalable and cost effective as it uses graphite powder and aluminium sheets as the raw materials. The presence of graphene layers in the metal matrix was confirmed using Raman spectroscopy as well as TEM. The graphene sheet thickness was measured using AFM after extracting it from the composite. Molecular dynamic simulation results reveal the evolution of newer structures and defects that have resulted in the enhanced properties of the nano-composite. These findings open up newer possibilities toward efficient and scalable manufacturing of high-strength high-ductility metal matrix based graphene nano-composites.

In almost all practical situations, graphene based nanodevices are subjected to complex
loading i.e., combination of shear and tensile stress. Given this situation, mixed-mode
fracture is inevitable during tearing of graphene. However, most of the studies on graphene
fracture are based on mode-I fracture which is an idealistic situation and rarely occurs
in the service conditions. We, therefore, performed classical molecular dynamics (MD)
simulations on a graphene sheet with crack like flaw and investigated the complex mixedmode
fracture behavior. Mode-I, mode-II, and mixed-mode stress intensity factors (KI , KII ,
and Keff respectively) as a function of  and crack length in armchair and zigzag edges were
calculated. In addition, we investigated the effect of slit length and angle on the strength
of graphene sheet. Effective stress intensity factor increases with flaw size and reaches a
plateau (between 3.10 and 3.80 MPapm for armchair, between 2.60 and 3.10 MPapm for
zigzag) approximately at a crack length of a/b ⇡ 0.11 (2a and 2b are crack and model size
respectively). For crack with zigzag edge surface, existence of a single bond perpendicular
to crack direction facilitates bond-breaking process. While for armchair surface case, two
inclined bonds at crack tip offer relatively more resistance. Finally, the effect of mixedmode
loading on the crack propagation path was investigated. All the systems considered in
this study mimic real service conditions. Hence, our findings will provide useful guidelines
for the design of graphene-based nanodevices.

In this work, we investigate the pressure and density characteristics of water film when simulated using the emerging technique called many body dissipative particle dynamics method. This work also layout the methodology of estimating local pressure from LAMMPS simulation using Harasima scheme. Using the triangular shaped cloud interpolation function, pressure and density are estimated at local bins and compared with the experimental database. Our results show good agreement for the molecular dynamics results of the argon system, while the many body dissipative particle model fails to simulate the water properties at room temperature. In its current form, the many body dissipative particle method cannot be used for accurate liquid vapor interfacial simulations and heat transfer studies.

In this study, classical molecular dynamics with the well-known van Beest, Kramer and van Santen potential are used for the first time to investigate the solid thermal conductivity of silica aerogel. Aerogel samples at various densities are obtained through negative pressure rupturing of dense silica samples, and reverse non-equilibrium molecular dynamics is employed to determine the thermal conductivity at each density. Results indicate that a power-law fit of the thermal conductivity obtained varies almost linearly with density, where decreasing density and increasing porosity led to an almost linear decrease in thermal conductivity. This is reflective of the trend observed in experimental bulk sintered silica aerogel. The results also showed that the thermal conductivity is of the same order of magnitude as bulk sintered aerogel. The power-law fit of the results also accurately reflected the variation found in bulk sintered aerogel.

Classical molecular dynamics with the AIREBO potential is used to investigate and compare the thermal conductivity of both zigzag and armchair graphene nanoribbons possessing various densities of Stone-Thrower-Wales (STW) and double vacancy defects, within a temperature range of 100-600 K. Our results indicate that the presence of both kinds of defects can decrease the thermal conductivity by more than 80% as defect densities are increased to 10% coverage, with the decrease at high defect densities being significantly higher in zigzag compared with armchair nanoribbons. Variations of thermal conductivity in armchair nanoribbons were similar for both kinds of defects, whereas double vacancies in the zigzag nanoribbons led to more significant decreases in thermal conductivity than STW defects. The same trends are observed across the entire temperature range tested.

Grain boundaries (GBs) as typical defective graphene structure have significant influence
on its mechanical properties. The fracture strength of hydrogenated graphene with tilt
GBs composed of pentagon–heptagon defects are systematically investigated using classical
Molecular Dynamics (MD) method. Anomalous mechanical characteristics are revealed
for graphene with hydrogenation either on or near the GBs. For graphene sheets with
hydrogenation on tilt GBs under perpendicular pulling, the strength of hydrogenated GBs
with large tilt angle is anomalously stronger than low-angle tilt boundary having fewer
defects because of the interaction between polar stress fields of hydrogenated pentagon–
heptagon defects. For graphene with hydrogenation near the GBs, the interaction between
GBs and hydrogenated domains at different distance intervals is investigated. The strength
is found to be governed by the peak normal stress in hydrogenated domain, and an
exponential relationship between the normal strength and distance interval is revealed
as a result of the stress field overlap between GB and hydrogenation interface. Our results
gain meaningful insight into the effects of hydrogenation on the strength of graphene with
tilt GBs, and provide guidelines for designing high-quality hydrogenated graphene-based
nanodevices.

The effect of glass stability on the yielding transition and mechanical properties of periodically deformed binary glasses is investigated using molecular dynamics simulations. We consider a binary mixture first slowly cooled below the glass transition temperature and then mechanically annealed to deeper energy states via small-amplitude oscillatory shear deformation. We show that upon increasing glass stability, the shear modulus and the yielding peak during startup continuous deformation increase towards plateau levels. It is found that during the strain amplitude sweep, the yielding transition occurs at higher amplitudes and it becomes more abrupt in deeply annealed glasses. The processes of initiation and formation of a shear band are elucidated via the spatiotemporal analysis of nonaffine displacements of atoms. These results are important for thermo-mechanical processing of highly stable amorphous alloys.

Graphene oxide (GO) nanoparticle is a high potential effective absorbent. Tetracycline (TC) is a broad-spectrum antibiotic produced, indicated for use against many bacterial infections. In the present research, a systematic study of the adsorption and release process of tetracycline on GO was performed by varying pH, sorption time and temperature. The results of our studies showed that tetracycline strongly loads on the GO surface via p-p interaction and cation-p bonding. Investigation of TC adsorption kinetics showed that the equilibrium was reached within 15 min following the pseudo-second-order model with observed rate constants of k 2 = 0.2742-0.5362 g/mg min (at different temperatures). The sorption data has interpreted by the Langmuir model with the maximum adsorption of 323 mg/g (298 K). The mean energy of adsorption was determined 1.83 kJ/mol (298 K) based on the Dubinin-Radushkevich (D-R) adsorption isotherm. Moreover, the thermodynamic parameters such as DHu, DSu and DGu values for the adsorption were estimated which indicated the endothermic and spontaneous nature of the sorption process. The electrochemistry approved an ideal reaction for the adsorption under electrodic process. Simulation of GO and TC was done by LAMMPS. Force studies in z direction showed that tetracycline comes close to GO sheet by C 8 direction. Then it goes far and turns and again comes close from amine group to the GO sheet.

The understanding of crack formation due to applied stress is key to predicting the ultimate mechanical behavior of many solids. Here we present experimental and theoretical studies on cracks or tears in suspended monolayer graphene membranes. Using transmission electron microscopy, we investigate the crystallographic orientations of tears. Edges from mechanically induced ripping exhibit straight lines and are predominantly aligned in the armchair or zigzag directions of the graphene lattice. Electron-beam induced propagation of tears is also observed. Theoretical simulations account for the observed preferred tear directions, attributing the observed effect to an unusual nonmonotonic dependence of graphene edge energy on edge orientation with respect to the lattice. Furthermore, we study the behavior of tears in the vicinity of graphene grain boundaries, where tears surprisingly do not follow but cross grain boundaries. Our study provides significant insights into breakdown mechanisms of graphene in the presence of defective structures such as cracks and grain boundaries.

Organic photovoltaics (OPVs) are a topic of extensive research because of their potential application in solar cells. Recent work has led to the development of a coarse-grained model for studying poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blends using molecular simulations. Here we provide further validation of the force field and use it to study the thermal annealing process of P3HT:PCBM blends. A key finding of our study is that, in contrast to a previous report, the annealing process does not converge at the short time scales reported. Rather, we find that the self-assembly of the blends is characterized by three rate dependent stages that require much longer simulations to approach convergence. Using state-of-the-art high performance computing, we are able to study annealing at length and time scales commensurate with devices used in experiments. Our simulations show different phase segregated morphologies dependent on the P3HT chain length and PCBM volume fraction in the blend. For short chain lengths, we observed a smectic morphology containing alternate P3HT and PCBM domains. In contrast, a phase segregated morphology containing domains of P3HT and PCBM distributed randomly in space is found for longer chain lengths. Theoretical arguments justifying stabilization of these morphologies due to shape anisotropy of P3HT (rod-like) and PCBM (sphere-like) are presented. Furthermore, results on the structure factor, miscibility of P3HT and PCBM, domain spacing and kinetics of phase segregation in the blends are presented in detail. Qualitative comparison of these results with published small-angle neutron scattering experiments in the literature is presented and an excellent agreement is found.

Porous structures of silica aerogels are generated using classical molecular dynamics, with the Tersoff potential, which has been re-parametrized for modeling silicon dioxides. This work demonstrates that this potential is superior to the widely used BKS potential in terms of characterizing the thermal conductivities of amorphous silica, by comparing the vibrational density of states with previous experimental studies. Aerogel samples of increasing densities are obtained through an expanding, heating and quenching process. Reverse non-equilibrium molecular dynamics is applied at each density to determine the thermal conductivity. A power-law fit of the results is found to accurately reflect the power-law variation found in experimental bulk aerogels. The results are also of the same order of magnitude as experimental bulk aerogels, but they are consistently higher. By analyzing the pore size distribution on different simulation length scales, we show that such a disparity is due to finite sizes of pores that can be represented, where increasing simulation length scales lead to an increase in the largest pore size that can be modeled.